Field of the Invention
The present invention relates to a method for manufacturing a laminated or stacked metal foil for joining laminated layers of a metal foil together through resistance welding.
Description of Related Art
To date, as sealed cells or batteries such as lithium ion batteries, those have been known in which an electrode body is formed by winding a plurality of times a separator and metal foils, which are an aluminium foil coated with a positive-electrode active material and a copper foil coated with a negative-electrode active material, for example. In such laminated layers of each metal foil, the site (uncoated portion) not coated with an electrode active material serves as a joint portion (weld site) for joining the metal foil with an electrode terminal, to be used as a current collection portion.
For lithium ion batteries, realization of higher battery capacity and avoidance of increase in the size of the battery exterior are demanded. In this case, means for increasing the capacity efficiency without changing the size of the battery exterior is exemplified by decreasing the width of the portion uncoated with the electrode active material in each metal foil to increase the area of the portion coated with the electrode active material.
Meanwhile, a strong insulating oxide film (aluminium oxide) is present on the surface of aluminium. Thus, it is conventionally known to provide a small hole in laminated layers of an aluminium foil serving as a positive electrode foil, by the use of a puncturing needle, for example, and then to perform resistance welding (Patent Document 1, for example). By providing the small hole, the oxide film of the aluminium foil is partially removed to make a path for current to flow therethrough, whereby conducting current performance or energization performance during resistance welding is improved.
On the other hand, with respect to laminated layers of a copper foil serving as a negative electrode foil, there are two kinds of the copper foil, i.e., rolled copper foil and electrolytic copper foil. The rolled copper foil is usually manufactured by repeating rolling/annealing of electrolytic copper into a foil shape. The electrolytic copper foil is manufactured in the following manner: performing energization in a copper sulfate vessel using a rotary drum made of stainless steel or the like as a cathode, to cause copper to precipitate (be electrodeposited) on the rotary drum; and then winding the precipitated copper into a roll shape.
From the viewpoint of the copper foil manufacturing methods above, the rolled copper foil has limitation in the length of the foil, but the electrolytic copper foil does not have such limitation. Thus, for a lithium ion battery, copper foil that does not have limitation in the length of the foil is desired in order to realize higher capacity. In recent years, the electrolytic copper foil is often used as the material for an electrode for a lithium ion battery.